This invention relates to preparing optical fibers having optical properties that can be changed by exposure to actinic radiation.
Optical fibers feature a core that guides the light and a cladding that confines the light. The performance of the fiber as an optical waveguide depends on the core and cladding refractivities which are controlled through the choice of materials used to fabricate the core and cladding.
Optical fibers are made primarily of ultrapure silica. Often, the fibers are doped to render them photosensitive. The property of photosensitivity permits photoinducing permanent changes in the fiber's refractivity after it has been fabricated such that optical devices are created in the fiber. Examples of such optical devices include Bragg gratings used to reflect certain wavelengths.
The reflectivity of a photoinduced Bragg grating depends on the fiber's photosensitivity and the time of exposure to the photoinducing light. Photosensitivity can be increased by using high concentrations of traditional dopants, e.g. germanium, but this has the disadvantage of reducing the diameter of the light travelling in the fiber, thereby causing signal loss via scattering at splices between photosensitive and industry standard fibers. Alternatively, low concentrations of traditional dopants and long exposure times can be used, but this is often impractical or results in gratings that are only moderately reflective.
Another source of signal loss in fibers containing Bragg gratings is the interaction between the light in the core and light in the cladding. Interference between light propagating through a core with a photoinduced grating and light in an unaltered cladding causes scattering of light and leads to significant signal loss out the side of the fiber (also known as "cladding mode losses"). Even in fibers containing photoinduced gratings in both the core and the cladding, significant side losses can result due to interference.